1. Field of the Invention
This invention relates generally to the attachment of thin pliable sheet material, such as plastic film, to a support, such as an enclosure frame work. More particularly, the invention relates to an improved sheet gripping assembly for this purpose and which is capable of securing either single sheet or dual sheet material.
2. Prior Art
As will appear from the later description, the improved sheet gripping assembly of the invention is capable of diverse sheet gripping applications involving a variety of thin pliable sheet materials. However, the gripping assembly is intended primarily for use in the construction of greenhouses of the class which comprise plastic film or other thin sheet material covering and supported by an open frame structure. The invention will be described in connection with this particular application.
The type of greenhouse referred to has an open frame structure constructed from pipes, tubes or other similar frame members. Covering and secured to this frame structure are plastic sheets or films which form a weather and wind resistent covering or skin transparent to the sun's ultra-violet radiation.
One essential requirement of such a greenhouse construction is firm but releasible attachment of the plastic film to the frame structure, such that the film will remain firmly in place under relatively high winds and yet may be quickly and easily replaced when necessary. A variety of sheet attachment devices have been devised for this purpose. One widely used type of attachment device is a sheet gripping assembly having a channel-like base to be firmly attached to the greenhouse frame structure and a removable sheet gripping member insertable laterally into the channel of the base through an open side of the channel. In use, the sheet gripping member is removed and the plastic film is placed across the open side of the base and then pushed through its open side into the channel to form a fold in the channel. The gripping member is then inserted through the open side of the channel to a position within this fold wherein the plastic film is firmly gripped between the base and gripping member.
Examples of this type of sheet gripping assembly are described in my prior U.S. Pat. No. 4,103,401 issued Aug. 1, 1978 and in my pending applications Ser. No. 06/462,468 filed Jan. 31, 1983 and Ser. No. 06/738,276 filed May 28, 1985.
Another general type of sheet gripping assembly for greenhouse constructions of the kind described has a base channel to be secured to the greenhouse frame structure and a channel shaped cap which snaps over the base channel. The plastic film to be secured is placed over the base channel after which the cap is pressed onto the base channel to firmly grip the film between the base channel and cap. One existing sheet gripping assembly of this kind is the sheet gripping assembly marketed under the trade name AGRA LOCK. Another similar sheet gripping assembly is that marketed under the trade name KWIK-LOC and described in U.S. Pat. No. 4,231,141. The present invention relates to this type of sheet gripping assembly and overcomes certain deficiencies of the existing gripping assemblies. These deficiencies reside in their tendency to weaken their grip on or release the sheet material when the latter is stressed in edgewise tension due to wind loads and thermal expansion of the air between the two sheets of two-ply material. At least one existing gripping assembly has an upwardly opening trough-like formation which is prone to be filled-up with dirt, leaves, and other debris.